1. Field of the invention
This invention relates to carbon fibers essentially consisting of monofilaments having a large diameter. Further the present invention relates to a liquid crystal pitch based continuous monofilament or continuous carbon fibers essentially consisting of small number of monofilaments. In addition, the "liquid crystal pitch" of the present invention is a general term for the pitch containing an optically anisotropic component and the pitch which is convertible easily to optically anisotropic by heat or stress.
The carbon fibers essentially consisting of large diameter monofilaments can be used for electric conducting material (heat-generating body, electrode material) electromagnetic-wave-shielding material, electric charge-preventing material, heat-resisting material, chemicals resisting material (filtration cloths, working cloths, protecting cloths, protection material, heat insulating materials), etc. Conventional carbon fibers have a drawback of being liable to form fluffs because their constituting monofilaments are thin and the number of the monofilaments is large and the carbon fibers are inconvenient to be treated as raw materials for such industrial materials.
Carbon fibers essentially consisting of large diameter monofilaments take a firm stand and have an advantage of low deformation due to the flow of the matrix component.
Even in the case where matrix materials have an extremely large surface tension such as metals, or have an extremely high viscosity such as certain kinds of thermoplastic resin, the carbon fibers of the present invention have small tendency to move and cause localization by the flow of matrix component. Accordingly, they show superior performance as a reinforcement material for fiber composite materials.
Carbon fibers consisting essentially of a small number of monofilaments, particularly a monofilament show superior performance as a core material of ceramic fibers, etc. produced by way of CVD (Chemical Vapor Deposition) to coat with boron, silicon carbide, silicon nitride, etc.
2. Prior Arts
In case of brittle materials like glass, it is known that strength becomes greater when they are shaped extremely finely. This has been explained from the fact that flaws are formed at a certain probability due to strain, etc. at the time of shaping, but if shaped finely, the probability of the existence of flaws included in the test piece becomes smaller, namely the finer the shaped body is, the greater the strength is improved.
It has been reported heretofore that such a phenomenon exists in case of conventional carbon fibers. In the case of PAN based carbon fibers, since strength becomes greater as diameter of fibers becomes smaller, thinner fibers are prepared in the course of time.
In case of isotropic pitch based carbon fibers, Otani investigated the relation between fiber diameter and strength; he reported that if diameter becomes greater than 10 .mu.m, strength drops suddenly [Carbon 3, 31-38 (1965)]. In case of mesophase pitch based carbon fibers, D. M. Riggs and J. G. Venner reported that the carbon fibers carbonized at 1500.degree. C. or 2000.degree. C. have a strength greater than 450 KSI when a fiber diameter is 8 .mu.m or less, and as the fiber diameter becomes greater, strength abruptly falls and at a diameter of 13.5 .mu.m the strength becomes 250 KSI or less (16th Biennial conference of Carbon).
From such a status, it has not been heretofore expected to produce a liquid crystal pitch based carbon fiber monofilament particularly a continuous monofilament.
Heretofore, synthetic fiber monofilaments have been produced by a process in which, after a spinning solution is extruded from a spinning nozzle and is solidified, stretching, heat treatment is successively carried out, and further fibers are continuously wound up after dividing into every monofilament or every several monofilaments.
This method is simple and does not require high grade of art but accuracy of quality of products are superior and this method is used for many synthetic resins such as, polyamide, polyester, polyolefin, etc.
Further in case for fibers having a large strength and elongation, such as polyamide, a method is also used in which filament yarns having relatively large monofilament denier is wound up while being detwisted and divided one by one or two by two monofilaments.
In case of pitch fibers, the strength of monofilament after spinning (extrusion) is drastically low and hence it is difficult to adopt these methods. The pitch fibers after spinning are wound up immediately after collection in bundles or their being picked up in cans or on a porous belt. The pitch fibers are subjected to infusibilization and carbonization while being kept in the state of being;wound up on a bobbin, in the state accommodated in cans or on a belt to avoid injury. It is natural that rough working such as fiber dividing cannot be put into practice unless the fibers are in the state of advanced carbonization. But even after carbonization it is extremely difficult in the point of strength as well as technique to divide the carbon fibers in which a large number of fine monofilaments are collected in bundles and carbonized into continuous monofilaments.